ZX81CCP video output for ZX81

[stefano]

Got the board and mounted it this morning.. now (after also cutting away part of the membrane stripes) my TS1000 is fixed !
The ZX80 port is at a good stage, the last problem to solve is to build a keyboard reading function able to leave the display visible, then some new program for the 4K ROM will see the light .

[PokeMon]

So finally a new release of the ZX81CCP - it's named ZX81CCB with same crystal clear picture quality but "basic" components. So a cheap but good picture version.
The schematics:

ZX81CCB_schema.gif (17.04 KiB) Viewed 6562 times

The main part is the powerful output driver for a sharp, crispy picture as known before. I used the same LVC technology from Texas Instruments. There is no more DIP switch but possible to choose the inverted picture by setting a sold jumper (J1). The two xor gates wired parallel looks a bit strange but there should be used either one of the two. The 74LVC1G386 is not more expensive but harder to get so I wanted to leave an alternative with a normal 74LVC1G86. On the other hand I have about 50 of these gates by now which can be used from me. Instead of the second AND gate which is used only as a driver there could be also used an OR gate (74LVC1G32).

ZX81CCB_board.png (14.4 KiB) Viewed 6562 times

The board is now very small with about 0.75 x 0.75 inches.

I will let produce a series of these boards which should be available in about 6 weeks. The price will be GBP 4.90 completely soldered and tested. If somebody will attend the German ZX Meeting I will take some boards with me. So you could spare shipment.

[sirmorris]

Brilliant stuff!!
I will have one or two at the meeting!

C

*edit - definitely 2

[Baggers]

These are awesome devices I recently got a ZX81 ( issue 1 ), but it unfortunately had a really bad RF signal, you could only barely make out that the ZX81 itself was indeed working!
Wired one of these up in seconds, and hey presto, crystal clear display and composite! worked a treat

Thanks!

[stefano]

Yes, I guess I should say thank you to Pokemon too for the invention, it saved my ZX81 from rubbish.. no, I'm joking, could never trash it , even if it was broken !
The CCP circuit is awesome.. is it normal to see it (quite rarely to be honest) flicker while some text is being printed ?

Hope the zx80 inclusion in z88dk will be appreciated, I'm crossing my fingers, can't wait to know if the software trick to keep the picture stable will work on the real hardware.

[PokeMon]

The CCP circuit is awesome.. is it normal to see it (quite rarely to be honest) flicker while some text is being printed ?

I know it is not really 100% flickerfree but this doesn't come from the CCP circuit as it just improves the signal delivered from the ULA.
I think this has to do with the key processing but didn't get very deep into it.
I am quite sure that it will flicker with other solutions (simple transistor output/discoupling) same way.

Maybe this effect is not seen via modulator but this is another way of processing incoming RF data.
Modern TV's have many filters in processing modulated video via RF.
Maybe that's the reason for not beein so obviously via RF.

I plan a new module 100% flickerfree with a buffered picture (frame) which would be suitable for ZX80 machines (keep the last picture until new picture is sent).
For now it is in my mind only but will come true with first tests in about 4 weeks.

[stefano]

Well the CCP trick seems already magic to me
But I confirm the latest TV models get in trouble when you issue a LOAD command because they prefer to squelch the output when they do not understand the TV signal fully (which does not happen with an older CRT TV set).

[PokeMon]

Well I developed the ZX81SCP for it - same as ZX81CCP but has an internal standby sync generator which produces a valid sync signal during LOAD command.
This is not working for the save command as there are created additional sync pulses internal from ULA but LOAD is more used than SAVE.

See this posting:
viewtopic.php?f=6&t=840&start=40#p9981

And the Youtube video for having the well known stripes from LOAD on a modern TV (LCD or PLASMA).

http://www.youtube.com/watch?v=TKNu3kavPl0

[stefano]

I saw your post but did not understand it fully, thanks for the video, I could compare the different shape of the stripes

[dr beep]

So I was able to fix the ZX81 after the keyboard membrane was broken, but then my very first TV I bought in my childhood gave up with a spectacular bang and a lot of smoke. On any other TV in the house I can't get a clear picture. Is this board the solution to connect with another old aerial TV?

If so, does it need additional soldering? And please can someone test the latest version of Pinball on a real ZX81 since my picture just doesn't get clear on the alternative TV.
viewtopic.php?f=6&t=1078&start=30#p11282